An electrically operated device that functions in a cordless mode typically is powered by a removable battery pack. The battery pack completely encloses one or more cells and provides the necessary DC power for operation of the device. Historically, cordless electrically powered devices have included relatively low power devices such as shavers and hand-held calculators. Recent improvements in battery technology have led to the development of batteries that store more energy and are capable of driving higher power devices. These devices include for example, portable hand-held power tools and appliances operating at power levels from 50 watts up to hundreds of watts. Using cordless power devices permits work operations to be performed in areas where a conventional AC power source is not available or is inconvenient to use. However, the use of cordless devices is limited by the effective charge capacity of the battery pack and the availability of replacement battery packs. When the battery pack is discharged, it must be recharged or replaced with a fully charged pack. Therefore, to compensate for the limited operating duration; extra battery packs or an optional corded AC converter module must be used with the cordless power device, or a corded power tool must be provided.
Both batteries and battery chargers are expensive in comparison to the power device for which they are intended. Batteries for high power applications cost approximately 30% of the cost of the applicable power device. Additional batteries are required to permit cordless mode operation while a battery is recharged and to replace dead batteries. High power levels drawn from batteries during operation of the power tool, the depth of discharge of the battery, the number of charge/discharge cycles, and the speed with which a battery is recharged all contribute to shortening the usable lifetime of a battery. To recharge batteries either a fast charger or a trickle charger must typically be used. A fast charger can be a significant portion of the cost of the power tool or appliance that is powered by the battery. A trickle charger is significantly less expensive than a fast charger, however a trickle charger requires approximately 1/2 day to recharge a battery pack. A fast charger on the other hand can recharge a battery pack within approximately one hour or less. Therefore, a trade off must be made between using a trickle charger with a large number of battery packs versus using a costly fast charger with very few replacement battery packs.
An optional corded AC converter module has only recently been provided for portable cordless power tools. The AC converter module connects to an AC power source and is designed to be interchangeable with the battery pack. The corded converter module converts power from the AC source to a regulated low-voltage DC level that is usable by the motor of the power device. Such a device allows a tool operator to use the tool in either the cordless battery mode or the corded AC mode as needed. Thus, the availability of such a device enables the operator of a cordless tool to complete a project when the battery pack has been discharged, or to continue to use the tool while the battery pack is charging and a fully charged backup battery pack is unavailable. Hence, by using a corded converter module the need for extra battery packs is reduced.
To increase the desirability of a corded converter module over the choices of additional battery packs or a separate corded power device, it is necessary to provide the desired output power capability at the lowest possible cost while maintaining a high quality device. The cost of a corded converter module is strongly related to the output power capability of the converter module. The higher the output power capability, the higher the cost. Therefore, it is desirable to design the output power capability of the corded converter module to be comparable to the output power capability of the corresponding battery pack. In addition, the maximum envelope of a corded converter module must conform to the envelope of the battery pack with which it is interchangeable. With the introduction of cordless tools of 24 volts and greater, the envelope of a conventional corded converter module is adequate for supporting the power output levels required to drive power devices such as hand held power tools. In tool voltages of 18 volts and below, the smaller battery pack sizes pose a challenge to the designer of an equally powered chopper circuit. Therefore, the main constraints on the output power capability of a corded converter module are the goals of minimizing size and cost and increasing reliability. Previously, attempts to minimize the cost of corded converter modules have concentrated on matching the output power capability of the converter module to a given power tool power requirement and then minimizing the cost of the resulting converter module components. By designing the converter module for the minimum output power required to satisfactorily drive the power tool, lower cost electronic components can be chosen for the converter. However, merely selecting the lowest cost devices that will attain the desired output power capability typically only results in marginal cost savings.
To obtain significant cost savings it is generally necessary to eliminate components from the design of the corded converter module. In a previously filed application, the power transformer that is used in a conventional corded converter module to meet government safety requirements was eliminated (see U.S. application Ser. No. 09/458,285). Instead of using the power transformer to meet the safety requirements, a double insulated case was relied upon. Generally, the power magnetics including power transformers and power inductors are amongst the more costly components within a corded converter module. Typically, conventional corded converter modules use a power inductor in combination with an output capacitor to filter voltage that is applied to the power tool motor. The power inductor is typically a custom designed device that is bulky and expensive in comparison with the other components of the corded converter module. The filtered voltage from the power inductor is applied to the motor, which has an inductance that is inherent in the construction of the motor. Optimizing the design of the power inductor to match the desired output power capability merely provides marginal cost savings.
Operators of cordless power tools already faced with the cost of battery packs and battery chargers must also invest in expensive corded converter modules for their power tools. As an alternative many purchase a corded power tool to use in lieu of the cordless tool when an AC power source is nearby. Attempts to minimize the cost of corded conversion modules have been constrained by the cost of using power inductors in combination with an output capacitor to filter the voltage supplied to the motor. Obtaining further cost reductions by reducing the output power level of a corded converter module would result in under-powered power devices. While the prior art can be used to provide corded converter modules for a handheld power tool, it has not proven capable of providing low cost modules that are convenient to use.